This invention relates in general to mechanical power transmissions. More specifically, it relates to a low friction cable transmission that is compact, extremely stiff, exhibits low torque ripple and uses cable to mesh flange-less pulleys whose axes of rotation can intersect.
Cables have long been used to transmit power. Conventional block and tackle arrangements used in cranes is one common example. With the Industrial Revolution, the power to weight characteristics of leather belts were found to be advantageous over gear trains in the transmission of power from a single, large-power engine to multiple end-use machines. In modern times, electric generator-motor sets replaced leather belt transmission and belts and cables have been used infrequently. Some examples of modern cable drives include the cabled dentist drill and belt drives for recordplayer turntables, computer printers, and other electro-mechanical equipment.
In the field of teleoperators and robot transmissions it has been conventional to rely on gears, sometimes combined with cables, to transmit power from a power source, typically an electric motor, to an end use, typically a vise-grip mechanical hand. Gears have been used because they have provided a stiffness not usually associated with cables and because sets of meshing bevel gears were necessary to transmit rotary power where the axes of rotation were not aligned, as at a right angle "shoulder" connection or in a differential transmission used to simulate the twisting motion of the human wrist. Heretofore, while cable or belt differentials have been used, the output pulley has been displaced laterally from the axis of rotation of the input pulley. The need to have the pulleys aligned to avoid belt wear and hold the belt on the pulleys has resulted in the physical interference of the pulleys being a limiting factor on the compactness of the differential.
A robot arm widely used in the automobile industry is the "PUMA 560" manufactured by the Unimation Corporation. It uses gears to transmit power along a set of articulated links to a "hand" mounted at its end remote from the power source. The use of gears results in a heavy transmission which is costly to manufacture and has poor control over the force applied by the arm. The PUMA 560 is also not "backdrivable", that is, if the output becomes the power input, the transmission does not reverse the power transmission with good fidelity. An important reason is that gears suffer from backlash and/or high friction. If they are machined to close tolerances and are well designed, backlash can be controlled and reasonable efficiency levels can be achieved, but the cost of manufacture will become quite high.
Robot arms using cables to transmit power over distances, in a manner analogous to a cable dentist drill, have been manufactured by Microbot. This approach is also used in the MA22 manipulator illustrated and discussed by Vertut and Coiffet in "Teleoperations and Robotics Evolution and Development", Robot Technology, Vol. 3A (Prentice-Hall, Inc. 1986), pp 190-191. Taking the MA22 as an example of this prior art, a fixed shoulder block communicates to an arm through a differential gear drive. An elbow and hand are controlled through cables, but the reduction is accomplished in a counterbalancing block near the motors and far removed from the point of application of the power, at the elbow and hand.
In the same Vertut and Coiffet publication at pp. 191-194, they also describe and show an MA23M manipulator using block and tackles with pretensioned cables to produce a large mechanical advantage. This system is shown in a highly simplified schematic form in FIG. 1 of the present application. While this system recognizes certain advantages of cable transmissions over gear trains, such as high efficiency and the elimination of backlash, it has several significant disadvantages. First, it requires the use of a long, continuous length of high tensioned cable which, over this length, introduces an undesirable compliance due simply to the inherent compliance of the cable itself. This compliance is probably related to a second problem, position dependent changes in the resonant frequency of the transmission that appeared when automatic controls were applied to the transmission. Third, the reduction provided by the block and tackle arrangement is near the motor; a long length of heavy cable couples the block and tackle assemblies to the load. This creates a very high tension in a comparatively long cable and requires that supports sufficient to withstand this level of applied force must span a comparatively long distance. All of these factors introduce the risk of a failure, but also increase the mass and cost of the arm at its remote end.
With respect to pretensioning, it is also important to note that while pretensioning per se is known, the way one goes about it can produce quite different results. Specifically, most pretensioning systems place a resilient element in series with the cable. With the Vertut MA23M manipulator, at least the long length of cable itself acts as a spring in series with the cable. In other applications, such as the Roto-Lok.RTM. cable drive of the Trax company, there is an actual pretensioning spring in series with the cable. Any such series compliance reduces the stiffness of the transmission, and in fact precludes the construction of a highly stiff transmission.
Finally, one of applicants has attempted to produce a cable differential analogous to intermeshed bevel gears, but using two pulleys with smooth bevel surfaces having a groove found in the surface to carry a cable that couples the bevel pulleys. This arrangement did not work since the cable tended to be drawn into the gap between the pulleys and bind. More generally, while cables and belts carried on pulleys are well known to transmit power, heretofore it has been a well known design objective to avoid coupling pulleys that are not aligned so as to avoid the attendant wear and eventual failure of the belt which must constantly rub against the pulley as it is angled out of the plane of rotation of the pulley.
It is therefore a principal object of the present invention to provide a bi-directional cable power transmission which is highly stiff, exhibits low frictional losses and has good fidelity of force transmission.
Another principal object is to provide a transmission between two or more smooth surfaced rotating members that uses a cable as the meshing element and is sufficiently compact that the axes of rotation of the members can intersect.
A further object is to provide a compact three cylinder differential transmission with a cable meshing of the cylinders.
Another object of the invention is to provide a power transmission that is compact, has a good aspect ratio, and which can can be used to drive an arm that can interact with objects at any point along a link.
A further object is to provide a power transmission with the foregoing advantages where an entire cable circuit can be pretensioned from a single point.
A still further object is to provide a power transmission which requires that the highest compressive forces acting on the system be supported over only comparatively short distances.
Yet another object is to provide a cable transmission with all of the foregoing advantages that is readily adaptable to automatic servo-control, and which has a low cost of manufacture as compared to known transmissions.